Magnet structure



Dec. 27, 1938. L. B. CORNWELL MAGNET STRUCTURE Filed Jan. 15, 1937 ENTOR (I M a m ATTORNEYS Patented Dec. 27, 1938 PATENT OFFICE MAGNET STRUCTURE Lionel Brice Cornwell, Stamford, Conn., assignor to Cinaudagraph Corporation, Stamford. Conn., a corporation of Delaware Application January 13, 1937, Serial No. 120,369

2 Claims.

This invention relates to magnets, and particularly to permanent magnets providing magnetic flux through an air gap.

The object of the invention is to provide a magnet structure which for a given weight and size will give a maximum flux through its air gap and a minimum leakage at all other points, thereby utilizing with the utmost possible efii- .ciency the available magnet energy of the metal.

In the accompanying drawing illustrating the invention a Fig. 1 is a cross section of a permanent magnet providing an air gap of cylindrical formation adapted to receive the voice coil of a loud speaker or the like, and

Fig. 2 is a similar diagram illustrating for comparison the characteristics of conventional magnets.

' As shown in Fig. 1 the permanent magnet structure comprises the magnet metal in the form of a ring or annulus 5, and plates 6 and ,I and core 8 tightly pressed at one end in end plate I and free at the other end, leaving precisely dimensioned air gap A between'the outer surface 9 of the core and the inner surface ll! of the end plate 6. This air gap A is adapted to receive a vibratory coil such as the voice coil of a loud speaker, and it is necessary to concentrate sufficient flux through this gap to develop the desired response and power in the voice coil and properly operate the loud speaker or other device.

To develop this flux and at the same time to reduce the sizes of the magnet members and the amount of metal and cost of the magnet, this invention specially forms the magnet parts to minimize leakage and to increase the proportion of useful flux passing through air gap A.

To decrease the leakage which short-circuits from the ring 5 and the plate 6 over to the core 8 at F, the inner surface of the ring is concaved as shown at, 13 and the core 8 is cut away as at H on a taper increasing in depth to the point 15 where it curves outward as the under surface of an overhang leading to surface 9 of gap A. Such protrusion of "the core 8 outward from point [5 gives an overhang of both under surfaces approaching the air gap A and aids particularly in preventing excessive leakage from the under surface of the plate 6 to the core 8 around the air gap. At the same time the ring 5 by this construction is brought radially inward at the areas of contact with the end plate 6 and 1 so as correspondingly to shorten the flux paths through these plates. The outer surface 11 of the ring 5 is convexed (Cl. 17911'I) outward preserving the uniformity of the cross section of the magnet metal and reducing the peripheral reverse leakage G by lengthening leakage paths between plates 5 and I. The remaining lakage J is that which passes up and over through the air through plate 6 to core 8 around the gap A.

' In the magnet of Fig. 1 the permanent magnet annulus 5 is preferably formed of a material such as Nipermag having a high coercive force, for 10 instance 380 oersteds, at a remanence of 3,000 gausses. For best efficiency of this permanent magnet these values are taken corresponding to the maximum product of coercive force and remanence for this material and will of course 15 because as above noted it-should operate at a density of 3,000 lines per square centimeter. 25 This for the construction shown in Fig. 1 will give an overall diameter of 8.2 inches for the I magnet metal 5 with a total flux of 559,500 made up of a useful flux of 342,000 through air gap A and a leakage fiux of 217,500. Of this leakage 30 flux 59,000'lines are at J, 65,000 lines at F, and 93,000 lines at G. The overall leakage factor equals 1.64 and with the parts as shown in Fig. 1 required to give this flux of 342,000 lines at air gap A, the weight of the whole structure is about 30.8 pounds, the weight of the magnet annulus 5 being 11.8 pounds. 40

In Fig. 2 a common type of permanent magnet structure is shown with the parts lettered and numbered corresponding in general to the same letters and numbers in Fig. 1, but in each instance primed. If instead of the new structure 45 as exemplified in Fig. 1 this conventional form of permanent magnet were followed as illustrated in Fig. 2; with air gap A identical with air gap A, the parts proportioned for maximum efficiency must be increased in size in comparison with the Fig. 1 construction to give the same useful flux, 342,000 lines, in air gap A, assuming, of course, the 'same materials in the magnet parts. The overall diameter of the conventional'structure is 9.25 inches and the total weight is 45.1 pounds, 55

of which 13.1 pounds is in the magnet annulus I. In forcing the 342,000 lines through air gap A, the leakage at J is 59,000 lines, the same as the J leakage of Fig. 1, the leakage at F is 82,000 lines as compared with 65,500 at F, and the leakage at G is 118,000 lines in comparison with 93,000 lines at G in Fig. 1. This gives a total leakage of 269,000 for the conventional magnet of Fig. 2, requiring a total flux of 601,000 lines and a leakage factor of 1.76.

The efficiency at the Fig. 1 magnet is over 61% as against an efllciency of less than 57% for the magnet structure shown in Fig. 2, each of these magnets giving precisely the same air gap flux. With this higher efficiency the magnet of Fig. 1 combines a lighter weight and lower cost. There is a saving in weight and cost of all parts, including the end plates associated with the magnet casting in the complete structure. By virtue of the reduced weight and size smaller and lighter associated parts for the support of the magnet in the assembly of a loud speaker, etc. are required. The design is applicable to various sizes of air gaps and various densities of flux as desired in each individual case,

and the greater effectiveness of the magnet of Fig. 1 may be utilized either to lower the size, weight and cost of the magnet for a given flux or to give a greater density or more total flux for the same weight of metal used.

These advantages are attained by simple changes in the forms of the constituent parts. Where the core 8 of generally cylindrical form is modified, it is simply shaped with an undercut following for instance the taper I4 and curve I! to provide the re-entrantly shaped annular recess, leaving the gap edge 9 of the core as an annular overhang of somewhat the same order of magnitude as the axial extent o'fthe air gap.

Cooperating with the core the concave contour I 3 of the annulus 5 recedes away from the core at intermediate points and approaches the core at the ends with closer approach at the end of the annulus away from the gap so as to leave the upper end of the annulus spaced away from the gap to form the gap edge III of the plate 6 into a free overhang. In general this inward overhang of the end plate 6 at the air gap A will be comparable in extent to the overhang formed on the core. The radial concavity of the surface I3 is substantially equal to the radial concavity of the core recess. Then consonant with maintaining a uniform most emcient cross section of the magnetic material of annulus 5, the outer surface I! of this annulus is convex to conform generally to the concavity of the surface i3, the end plates 6 and I having their peripheral surfaces beveled to merge with the curvature of the surface ll. 'I'his outside curvature of the whole magnet structure lengthens the leakage path G as compared with G and thereby increases the reluctance of this path and consequently decreases the leakage.

There is nothing complicated in the shaping of the Fig. 1 magnet structure, the parts being easily cast to the desired contour or machined from circular cast forms where machining is feasible and preferable. In practice the material of the annulus 5, such for instance as Nlpermag", would be cast and ground because this material is difflcult to machine. Other parts may be machined. The leakage reducing formations of the core and annulus have been combined in the structure of Fig. 1 to bring out the cooperation between them, but these special contours may be advantageously used separately for instance by combining the bulged annulus formation with a straight cylindrical core without the special undercut.

The magnet structure of this invention thus attains the same or greater air-gap flux with smaller and lighter parts and at a substantial saving in cost.

I claim:

1. A permanent magnet structure comprising in combination, a core structure, a permanent magnet consisting of a substantially continuous ring of uniform cross section made of material having a high coercive force not less than 300 oersteds at a remanence of 3000 gausses and a radial thickness at least half the length of the magnet between poles, a low reluctance connection between said ring and the core structure at one pole and an end plate of low reluctance flux-concentrating metal at the other pole of said ring and forming an air gap with the corresponding end of the core structure, the inner surface of the permanent magnet ring being outwardly deflected adjacent the air gap to provide a lengthened leakage path around the inner edge of the gap, and the outer face of the ring being deflected outwardly beyond the outer margin of the end plate and of the opposite magnet pole to provide a ring of substantially uniform radial thickness and an elongated leakage path between the magnet poles.

2. A permanent magnet structure comprising in combination a core member, a surrounding annulus comprising a pair of circular end plates of low reluctance metal, and a substantially continuous permanent magnet ring of uniform cross section fitted between the end plates, the ring being constructed of permanent magnet metal of high coercive force not less than 300 oersteds at a remanence of 3000 gausses and a radial thickness at least half the length of the magnet between poles, one end plate being connected to the core member to provide a continuous low reluctance path for magnetic flux from the adjacent pole of the magnet to the core member, the other end plate forming an air gap with the core member, the inner surface of the permanent magnet ring being outwardly deflected adjacent the air gap to provide a lengthened leakage path around the inner edge of the gap, and the outer face of the ring being deflected outwardly beyond the outer margins of the end plates for a distance suillcient to compensate for the deflection of the inner face and provide a ring of substantially uniform radial thickness and an elongated leakage path between the end plate margins.

LIONEL BRICE CORNWELL. 

